Dew point detecting and ice preventing device



DEw POINT DETECTING AND IcE PEEVENTING DEVICE Filed Jan. 2. 1964 NOV 8, 1956 M. F. clEMocHowsKl 3 Sheets-Sheet l Nov. 8, 1966 3,284,003

DEw POINT DETECTING AND ICE PREVENTING DEVICE M. F. CIEMOCHOWSKI 5 Sheefs-Sheet 2 Filed Jan.

NOV- 8, 1966 M. F. clEMocHowsKl 3,284,003

DEW POINT DETECTING AND ICE PREVENTING DEVICE Filed Jan. 2 1964 5 Sheets-Sheet 5 Cl/PRF/V- VOL TA 6E ROP INVENTOR. MICK/Afl F. C/MOC0W5/(/ BY www, W

4 7" TOR/VE Y United States Patent C 3,284,003 DEW POINT DETECTING AND ICE PREVENTING DEVICE Michael F. Ciemochowski, Warren, Mich., assigner to Holley Carburetor Company, Warren, Mich., a corporation of Michigan Filed Jan. 2, 1964, Ser. No. 335,285 8 Claims. (Cl. 236-44) This invention relates generally to atmospheric condition indicating devices, and m-ore particularly tc an electronic type device for anticipating a dew p-oint condition and actuating a heat source to prevent condensation and ice formation.

In a number of situations, it is necessary to prevent the formation of lice on some particular surface. For eX- ample, a very undesirable and dangerous situation is created when ice is permitted to form at the compressor stage of a gas turbine engine. Another common example of this is carburetor icing, which may take place at atmospheric temperatures as high as 50 F.

I-t is recognized that p-rior art systems have been proposed to prevent the formation of ice on -certain surfaces. One type of prior art device senses condensed water vapor or so-called free water. However, a main ldisadvantage of this type -of device is .that no action to prevent formation of ice may be taken until after water susceptible to freezing is already present. Furthermore, in certain instances it may also be desirable to prevent condensation lon the surface in question.

Another prior art device of this -kind -operates in response to the increase in water vapor pressure as the dew point is approached. A problem with this type of device is that accurate water vapor pressure readings are diicult to isolate, since the vapor pressure includes vapor pressures of other substances present .in the atmosphere.

It is now proposed to provide means for preventing condensation of water vapor on a surface by means capable of predicting when the dew point or s-atu-ration condition is being approached. Obviously, since water vapor will not freeze and since the proposed device prevents condensation, it also prevents ice formation.

Very generally, the proposed device senses `a-ir temperature, temperature of the surface and relative humidity; then, by appropriate manipulation of related voltage signals by electronic means, an imminent dew point condition is anticipated so that heat may be supplied to the surface in question.

Accordingly, a primary object of this invention is to provide a novel means for anticipating atmospheric conditions likely to result in condensation of water vapor on some particular surface.

Another object Iof the invention is to provide such means responsive to atmospheric moisture content and temperature, and temperature of the surface.

Still another object of the invention is to provide such means for hea-ting said surface when condensation of water vapor thereon is imminent.

Another object of this invention is to provide such means including a capacitative moisture sensing element comprising a dry hygr-oscopic dielectric material which absorbs and gives up moisture almost instantly.

A more specific object of the invention is to provide such means wherein the capacitance moisture signal is changed into a m-ore convenient signal voltage form and added to .a second voltage which is proportional to ambient air temperature; the summated voltage is then divided by an appropriate scaling factor, producing a third voltage which is proportional to a dew point or saturation temperature. This later voltage is then compared with a fourth vol-tage which is proportional to the temperature "ice of lthe surface whereon condensation and ice should not form. Once the fourth voltage equals or fa-lls below the third voltage, a heat source is actuated, heating the surface involved and thereby preventing condensation and ice from forming.

Other objects and advantages of the invention will become more apparent when reference is made to the following specication and the acc-ompanying drawings wherein:

FIGURE l is a schematic illustration of the invention;

FIGURE 2 is `a side elevational view illustrating the structure of the moisture sensing element shown by FIG- URE l;

FIGURE 3 is a cross-sectional view taken along the plane of line 3 3 of FIGURE 2 and looking in the direction of the arrows;

FIGURE 4 is a fragmentary View taken along the plane of l-ine 4-4 of FIGURE 3 and looking in .the direc-tion of the arrows;

FIGURE 5 is a graph illustrating a characteristic of the element illustrated in FIGURES 2-4;

FIGURE 6 is a graph illustrating another characteristic of the element illustrated in FIGURES 2-4; :and

FIGURE 7 is a graph illustrating sti-ll .another characteristic -of the element illust-rated in FIGURES 2-4.

Referring now to the drawings in greater detail, FIG- URE l illustrates schematically the proposed condensation and ice prevention system 10. The particular system 10 shown comprises three sensing elements and a total of nine driving, amplification ,and reference stages. More specifically, the system includes a radio frequency crystal oscillator stage 12 which is connected to a source of direct current power by means of a lead 14. Since an oscillator will usually not provide sufficient power, la radio frequency amplifier stage 16 is coupled thereto by lead 18, in order to provi-de the required voltage to a capacitive moisture sensing element 20 through the lead 22.

An ambient air tempera-ture sensing thermistor 24 is inserted in the line 28 lbetween the direc-t current supply.

line 14 and a so-called summation logic circuit 26. The air .temperature sensing thermistor 24 may comprise a variable resistor having a positive temperature coeiicient and a voltage drop proportional to the ambient air temperature. The re-sultant ambient .air temperature voltage signal shall be hereinafter designated as V1.

The moisture sensing element 20, which will be -described in greater detail later, supplies a current signal, which is proportional at all times to they relative humidity factor, to the summation logic circuit 26 through lead 30. The circuit 26 converts the current signal to a voltage signal V2 and thereupon adds the temperature and moisture indicative voltages, V1 and V2, and supplies the resultant voltage, hereinafter designated as C, to a differential amplifier 32 via a lead 34.

The differential amplifier 32 serves as a switching stage, utilizing the moisture voltage signal V2 as a reference voltage, the latter being supplied thereto via lead 36. While the manner in which the actual switching arrangement is accomplished may vary, many different techniques .being well known in the art, one specific way in which this -could be accomplished would be to include a pair of transistors (not shown) in the `differential amplifier 32 and feed the reference voltage V2 to the base of one of the transistors through a zener diode (not shown) in the line 36.

Once the summated voltage signal C is received by the differential amplifier 32 from the summation logic circuit 26, it is then fed by the differential amplifier 32 to one of a pair of scaling -divider circuits 38 or 40 via lines 42 or 44, respectively. More specifically, one yof the divider cir- 23 cuits 38 or 40 would be normally connected to the switching stage 32 until su-ch time as the above mentioned reference voltage reaches a predetermined level. At that time, the second transistor in the switching circuit 32 would permit a signal to be fed to a suitable relay (not shown), which in turn would break the circuit normally connected to one of the scaling divider circuits 38 or 4t) and make the circuit with the other scaling circuit. While two scaling divider circuits wiil normally suice for most icing conditions, additional divider circuits may be added to cover a greater expected ice formation range.

The scale -divider circuits 38 and 40 serve to divide the summated voltage C by an appropriate scaling factor, K1 or K2. The resultant voltage signal, hereinafter called N1 or N2, obtained lby dividing C by K1 or K2, respectively, is indicative of the dew point or saturation temperature for a given relative humidity and air temperature. The voltage N1 or N2 is supplied to a differential ampliiier comparator 46 via leads 48 or 50, respectively.

The third sensing element, namely, a suitable temperature sensing thermistor 52, which may comprise a variable resistor (not shown) connecte-d in a line 54 between the line 28 and the comparator 46, senses the temperature of the surface (or surfaces) (not shown) whereon ice would normally form, but whereon it is desired that it not be permitted to form. The resultant output of the thermistor S2 is a voltage V3 which is communicated to the comparator 46 through the line 54. Thus, the comparator 46 receives two voltages, i.e., V3 and either N1 or N2, and when the surface temperature signal V3 becomes equal to or smaller than the saturation temperature signal N1 or N2, the comparator 46 will transmit a command signal A via a line 56 to any suitable device S8, such as a solenoid for actuating a source of heat. The latter, in turn, will supply heat to the surface in question, as indicated by line 60, thereby raising the surface temperature.

As the surface temperature is raised suiiiciently above the dew point or saturation temperature, the temperature sensing thermistor 52 will signal the comparator 46 accordingly, whereupon the latter will cle-energize the actuation device 58, shutting olf the heat supply therefrom.

While the comparator may consist of any one of a plurality of well known circuits, one such suitable circuit (not shown) may include a pair of transistors, the respective bases of which receive the V3 and the N1 or N2 signals, compare them, and signal the solenoid accordingly.

The two differential amplifier stages 32 and 46 require stable voltage references in order to function as intended. This is accomplished Iby the incorporation of a voltage reference signal supply stage 62 in the system. Leads 64 and 66 connect the stage 62 to the amplifier stages 46 and 32, respectively. The stage 62 may be connected to the power supply line 14 via a line 68, either directly or indirectly through the lines 54 and/or 2S. This stage 62 may, of course, consist of the usual Zener diodes and resistors (not shown).

MOISTURE SENSING ELEMENT The moisture sensing element 20, which is shown by FIGURES 24, includes a sheet of dielectric material 70 which, together with the pair of perforated electrode plates 72, provide a capacitor. At least one of the plates 72 is preferably spaced from the relatively thin sheet of dielectric material 70 to increase the free circulation of air therebetween, and all three of these elements are supported at the edges thereof Aby insulators 74. The insulators are conlined within a housing 76 having suitable mounting legs 78 attached thereto. The sides 80 of the housing 76 adjacent the perforations 82 (FIGURE 4) of the electrode plates 72 are open and may include wire screens 84. The perforations 82 and the screens 84 provide'access of air to and around the dielectric sheet 70 to enable a uniform moisture distribution therein.

i The electrode plates 72 may be made from polished stainless steel to alleviate the possibility of chemical reactions resulting from the interaction of environmental contaminants.

The basic operating principle of the capacitive moisture sensing element 20 concerns the changes occurring in the dielectric constant (specific inductive capacitance) of the selected dielectric material between its moist and dry conditions. A preferred material is polyvinyl alcohol, known as Elvanol, because it possesses one of the highest dielectric constants of any plastic material; being hygroscopic, its dielectric constant varies lineally with humidity. The following Table I shows these values for a particular grade of polyvinyl alcohol.

Additionally, polyvinyl alcohol will absorb moisture almost instantly and give it otf at substantially the same rate once it is exposed to a drier atmosphere. From the following Table II it may be noted that the gain in weight is stable at any humidity level.

Table II Relative humidity, Gain in weight, percent: percent 35 2.5

The above characteristics, along with a very high moisture vapor transmission rate, make this particular material ideally suited for use in moisture measuring elements.

Applying the basic capacitance equation of a typical parallel plate capacitor,

to the moisture sensing element 20, it is apparent that an increase in dielectric constant K results in a proportional increase in thecapacitance C (in pico-farads), inasmuch as the area A (in square inches) of the electrodes and the .thickness D (in inches) of the dielectric are constant terms. Y

Referring once again to Table I, we may conclude that the capacitance, which is proportional to the dielectric constant, is also proportional to the relative humidity level. This relationship is illustrated in FIGURE 5.

However, in order to utilize this change in capacitance as an indication of particular humidity conditions, it is necessary to develop a signal in some usable form, such as voltage or current. This may be accomplished by connecting the moisture sensing capacitor 20 to a source of alternating electromotive force, such as a transformer (not shown) in the ampli-tier stage 16, the specific connection being by the leads S6 from the electrode plates 72.

The applied voltage, e, changes direction continually, and the charge, q, in coulombs, on the capacitor 70 changes with it in accordance with the equation q=Ce, wherein C is the above mentioned capacitance. Thus,- the capacitor 70 is charged alternately in opposite direc tions due to the usual A.C. signal received from the radio frequency amplifier stage 16.

Since e=Em sin 0, where Emzpeak voltage, =phase angle (2 radians/cycle or 21rf radians/ second) and f=frequency in cycles per second, the current owing will be It will be recognized that this equation may be transformed to =21rfCEm COS 21rfl which shows that current leads voltage by 90.

Since the maximum value that a cosine can have is unity, I,n (peak amperage) =21rCEm.

Dividing both sides by \/2, I =21rfCE The factor 21rfCE represents a reaction due to frequency. Thus, the opposition to current is Therefore, the capacitive reactance, XC, is

From the above, it can be seen that as the capacitance C increases, the reactance Xc decreases, permitting a larger current to pass. Consequently, when the applied voltage and frequency are constant, the rise of current in the circuit will be proportional to the increase in capacitance.

Since the capacitance C rises lineally (FIGURE 5) and in view of the above formula, I=21rfCE, wherein the voltage and frequency are constant, the current I through the capacitor portion 70/72 of the element 20 will also rise lineally as a function of relative humidity, as illustrated in FIGURE 6.

The signal output must be manipulated through the summing circuit 26, and it is much more convenient to use a voltage output rather than amperage. Consequently, the alternating current may be vrectified by a full bridge rectifier (not shown) and passed through a suitable resistor or potentiometer included in the summing circuit 26, with a resultant voltage drop thereacross. The voltage drop, previously designated as V2, will then be proportional to the current passing through the resistor; therefore, it will be an indication of the relative humidity condition reflected in change of capacitance. FIG- URE 7 shows the current and the voltage drop across such a resistor, as plotted against the capacitance of sensing element 20.

As indicated, the relations of current, voltage, and capacitance are linear, permitting simple signal manipulation in the logic circuit 26. The previously mentioned addition of temperature and moisture indicative voltages, V1 and V2, may result simply by connecting the above mentioned resistor and the previously discussed temperature sensing thermistor 24 in series.

SUMMARY In the above description, various elements of the proposed dew point detecting and/or ice preventing system are represented schematically in the drawings and identified generally as radio frequency oscillator, radio frequency amplifier, summation logic circuit, temperature sensing thermistor, scaling divider circuit, differential amplier comparator and command, etc. In addition, the desired performance characteristics of these various componenents have been stated.

It is believed that the invention lies more in the combination of these various components to provide the system, rather than in the specific design of the components. That is, one skilled in the art would recognize more than one possible construction for a particular component to perform the function required in the combination.

Thus, referring to the lblock diagram of FIGURE l, it will be apparent that D.C. may be any source of direct current, such as a battery. `Component 12 may be any circuit and/or device that will convert the D.C. to alternating current (A.C.) and that component 16 may be any circuit -and/ or device that will amplify the A C. from 12. While components 12 and 16 are labeled Radio Frequency, other frequencies may be employed, depending upon the nature of the dielectric material and other pertinent factors. 4Component 24 may be any circuit and/or a device that will provide a D.C. output voltage proportional to air temperature and component 20 may be any circuit and/or device that will provide an output current proportional to relative humidity. Component 26 may be any circuit and/ or device that will (a) convert the A.'C. output current of 20 to D.C. current and then produce a voltage proportional to relative humidity, and (b) -add the D.C. output voltage of 24 to the D.C. volt` :age resulting from 20, this addition being the output of 26. Components 38 and 40 are any circuits and/ or devices that will drop the output voltage from 26 to a value substantially proportional to dew point temperature; for greater accuracy, additional similar circuits and/or devices may be used. Component 32 may be any circuit and/ or device that will compare the output voltage from 26 with the converted output voltage from 20 and then supply the output voltage from 26 to one or the other of the components 38 or 40, depending upon whether the converted output voltage is indicative of a relative humidity above or below a predetermined Value, say 50%. Component 52 may be any circuit and/or device that will provide a D.C. output voltage proportional to the temperature of a particular surface. Component 46 may be any circuit and/ or device that will compare the output voltage from 52 with the output voltage from either the 38 or 40 device and provide an output sufficient to operate some particular actuation device, such as a heat source, when the output voltage from 52 equals or drops below the output voltage from 38 or 40. Finally, component 62 may be any circuit and/or device that will maintain the reference points of the transistors contained in both 46 and 32 :at fixed voltage values.

Suitable circuitry for the above components may be found in any electrical handbook, such as the R.C.A. Transistor Handbook, Issue 1961.

The moisture sensing element 20 is described in greater detail. Here again, however, equivalent structures may be possible.

With proper selection of the components, the system disclosed will function efficiently -at standard, relatively constant sea level barometric conditions and can be made to withstand temperatures from 40 F. to 140 F. without adverse effects or resultant malfunction.

In the preferred embodiment, the scaling divider circuits would -be such that the system would function with suicient -accuracy within the probable ice formation range of 20 F. to 40 F. This range can be selected because the air is usually dry at temperatures below 20 F. and because no ice is likely to form in the intended application at temperatures above 40 F. However, the temperature range can be extended by selection of -appropriate scaling divider circuits, as previously indicated.

In view of the above description, it should be readily apparent that the invention represents a novel approach to so-called ice prevention systems; that is, the icing condition, and not the ice, is eiciently and automatically detected so that the ice is prevented from forming.

It is also apparent that the invention may be used to detect the dew point in Iany desired temperature range for purposes other than prevention of condensation and/ or icing. For example, it may be desired to spray la chemical or cause some other action when dew point is approached.

Although but one embodiment of the invention has been disclosed and described, it is apparent that other modifications lare possible within the scope of the appended claims.

What I claim as my invention is:

1. A device for predicting condensation of moisture on a surface, said device comprising means for producing a first voltage signal proportional to relative humidity, means for providing a second voltage signal proportional to ambient air temperature, means for adding said first and second voltage signals and dividing the sum thereof by an appropriate constant to produce a. third voltage signal proportional to dew point, means for producing a fourth voltage signal proportional to the temperature of said surface, and means for comparing said third and fourth voltage signals and signaling when said fourth voltage is substantially equal to or below said third voltage.

2. The device described in claim 1, wherein said dividing means includes a plurality of scaling divider circuits, eac-h operable Within predetermined adjacent temperature ranges.

3. A device for predicting condensation of moisture on a surface, said device comprising means for producing a rst signal proportional to relative humidity, means for providing a second signal proportional to ambient air temperature, means for adding said lirst and second signals and dividing the sum thereof by an appropriate constant to produce a third signal indicative of saturation temperature, means for producing a fourth signal proportional to the temperature of said surface, and means for comparing said third and fourth signals and signaling when condensation is likely to form on said surface.

4. An electronic device for preventing condensation of moisture on a surface, said device comprising -a source of direct current, means for converting said direct current to alternating current, means for amplifying the a1- ternating current, means for providing a direct current voltage output proportional to ambient air temperature, means for providing an alternating current output proportional to relative humidity, means for converting said alternating current proportional to relative humidity to a direct current and producing a direct current voltage indicative of relative humidity, means for adding said output voltages proportional to ambient air temperature and relative humidity, means for comparing said added output voltage With said voltage proportional to relative humidity and for dropping said added output voltage to a value substantially proportional to dew point temperature, means to provide a direct current output voltage proportional to the temperature of said surface, means for comparing said dropped voltage to said voltage proportional to the temperature of said surface and providing a signal when said dropped voltage is equal to or greater than said voltage proportional to the temperature of said surface, and means responsive to said latter signal for supplying heat to said surface.

5. An electronic device for preventing condensation of moisture on a surface, said device comprising a source of electricity, means for providing `a voltage output proportional to ambient air temperature, means for providing a voltage output proportional to relative humidity, means for adding said output voltages proportional to ambient air temperature and relative humidity, means for comparing said added output voltage with said voltage proportional to relative humidity and for dropping said added output voltage to a value indicative of dew point temperature, means to provide an output voltage proportional to the temperature of said surface, means for comparing said dropped voltage to said voltage proportional to the temperature of said surface and providing a signal when said dropped voltage is equal to or greater than said voltage proportional to the temperature of said surface, and means responsive to said latter signal for operating an actuation device.

6. The device described in claim S, wherein said means for dropping said added output Volt-age includes a differential amplier and an associated scaling divider circuit.

7. The device described in claim 5, wherein said means for dropping said added output voltage includes a differential amplifier and an associated plurality of scaling divider circuits.

8. The device described in claim 7, wherein each of said plurality of scaling divider circuits operates within a predetermined temperature range.

References Cited by the Examiner UNITED STATES PATENTS 2,047,638 7/1936 Kott 73-3365 2,403,917 7/1946 Gille 236-1 2,687,342 8/1954 Strange et al. 236-44 X 2,813,235 11/1957 Clay 236-78 X 2,943,488 7/1960 Strobel et al. 73-336.5 2,974,870 3/1961 Pitts 236-44 3,056,935 10/1962 Jensen 338-35 3,070,301 12/ 1962 Sarno. f 3,168,829 2/1965 Nelson 73-3365 3,181,791' 5/1965 Axelrod 236-44 ALDEN D. STEWART, Primary Examinez'. 

1. A DEVICE FOR PREDICTING CONDENSATION OF MOISTURE ON A SURFACE, SAID DEVICE COMPRISING MEANS FOR PRODUCING A FIRST VOLTAGE SIGNAL PROPORTIONAL TO RELATIVE HUMIDITY, MEANS FOR PROVIDING A SECOND VOLTAGE SIGNAL PROPORTIONAL TO AMBIENT AIR TEMPERATURE, MEANS FOR ADDING SAID FIRST AND SECOND VOLTAGE SIGNALS AND DIVIDING THE SUM THEREOF BY AN APPROPRIATE CONSTANT TO PRODUCE A THIRD VOLTAGE SIGNAL PROPORTIONAL TO DEW POINT, MEANS FOR PRODUCING A FOURTH VOLTAGE SIGNAL PROPORTIONAL TO THE TEMPERATURE OF SAID SURFACE, AND MEANS FOR COMPARING SAID THIRD AND FOURTH VOLTAGE SIGNALS AND SIGNALING WHEN SAID FOURTH VOLTAGE IS SUBSTANTIALLY EQUAL TO OR BELOW SAID THIRD VOLTAGE. 